In the past, UV-transmission through silica based multimode-fibers with an undoped synthetic core and fluorine doped cladding has been unsatisfactory due to generation of UV defects. Such UV fibers have been used for light transportation mainly with deuterium lamps, excimer-lasers at 308 nm, 248 nm and 193 nm, frequency doubled argon lasers and fourth harmonic Nd-YAG lasers.
Due to material modification (e.g. reduction of chlorine, increase of OH-ions, stoichiometric deposition) and due to hydrogen loading at high pressure (see German Patent DE 195 47 904 A1), it was found that the UV-damage could be significantly reduced. In particular, E'-centers due to weak bonds or chlorine impurities could be passivated by hydrogen, shown in the improved UV-performance of the fibers. Especially, thick core fibers, having a core diameter greater than 300 .mu.m, have a long lifetime because the reduction of hydrogen content by outgasing decreases with increasing fiber diameter. The lifetime, defined as the time leading to a 3 dB/m induced loss at 214 nm, is initially linear with the cross-section area. However, for thinner fibers in a fiber bundle, the commercially available UVI(SR)-fibers from Polymicro Technologies, Inc. loaded with hydrogen do not have a sufficiently long lifetime.
The limited lifetime of UVI(SR) fibers loaded with hydrogen is due to the fact that hydrogen in UV-damaged material is not sufficiently bonded and migrates outwardly to diffuse from the fiber. The hydrogen diffuses out of the fiber, especially from thin fibers, e.g. from fiber bundles having fibers with a core diameter around 100 .mu.m and a cladding diameter of about 110 .mu.m or singlemode fibers having a core diameter about 10 .mu.m and a cladding diameter of about 125 .mu.m (typical values). In this case, the lifetime is restricted to the range of about one month or less, at room temperature. However, hydrogen reloading is possible leading to similar performance comparable with the performance after the initial hydrogen loading.
In previously known applications, a hermetic coating (carbon is mainly used) as a hydrogen barrier is used in the opposite direction to prevent indiffusion of hydrogen into silica fibers in order to avoid the generation of absorption bands in the NIR-region, which is used for optical communication in telecom applications (at 1.55 .mu.m lowest attenuation value of 0.2 dB/km). The hydrogen induced attenuation may be orders of magnitude higher than the basic attenuation, depending on the surrounding atmospheric pressure.
Similar results have been obtained with short length high-OH silica bulk samples loaded with hydrogen. The UV-transmission is found to be more stable in comparison to standard bulk material. Parallel studies have been carried out on low-OH material using a high temperature hydrogen treatment to modify the UV defects, leading to a material having OH-content of approximately 20 ppm. However, the hydrogen content in the samples, due to the hydrogen pressure used, is significantly lower than in the fiber of the invention described below.
Although an advantage of hydrogen in resistance against high-energy radiation or particles has been shown in silica bulk material and fibers, the previous studies were done with the incorrect loading parameters and without taking into account the outgasing of hydrogen during storage at room temperature or during testing, based on our new knowledge. On the other hand, most of the radhard-studies have been done in the wavelength region above 400 nm (VIS- or IR-region) because the UV-damage seemed to be too severe.
The role of hydrogen for radiation-improved material in the VIS- and IR-region has been described in several papers. However, no data are available concerning UV-performance in gamma-irradiated fibers with hydrogen, in comparison to irradiated fibers without hydrogen. Up to now, the influence of gamma-irradiation on UV-performance of UV-fibers, either singlemode- or multimode fibers having undoped silica core, with and without parallel UV-light transportation, has not been known.
The high-OH material has been studied in detail because of the well known UV-performance (because of residual hydrogen). However, low-OH undoped core material is attractive for the deep UV-region, because there is an indication that the UV-edge is shifted to shorter wavelengths. However, the drawing-induced UV-defects are so high that commercial usage of low-OH fibers in the deep U-region is not known.
Results of gamma-radiation on fiber transmission, mainly in the VIS- and IR-region, are summarized in the literature: SPIE-Proceedings Vol. 787 (Orlando 1986), Vol.867 (Cannes 1987), Vol. 992 (Boston 1992), Vol. 1174 (Boston 1989), Vol. 1799 (Boston 1992) or in /Grisc 1,2; Frieb 1; Lyons 1,2/. An excellent state of the art of radiation hard fibers in 1990 is described in "Large hadron collider workshop (Aachen, October 1990)" by G. Jariskog and D. Rein (editors)/Fabian 2/. In addition, a new paper concerning gamma-induced losses in the VlS-region point out that residual hydrogen may improve the fiber slightly (Grisc 3,4.)
In /Lyons 1/ the UV-transmission and the transient radiation sensitivity of standard UV-fibers have been compared; however, the U-transmission was only measured before treatment for the reasons described above.
No knowledge exists at the moment concerning UV-annealing in the UV-region, after gamma-damage, with a low-power deuterium-lamp or a high-power UV-laser starting below 350 nm due to two-photon absorption.